An MDF board measuring 400mm diameter and 18mm deep. Please note that these measurements are from outside edge to outside edge. The frame is 5mm wide, which means the internal dimensions of the board (the space available for your mosaic) will be slightly smaller than the listed size.  Just to be clear! The frame rises 5mm from the board surface and the depth of the entire board, including frame, is 18mm. This 5mm depth means that these boards form an ideal base for most of our glass and ceramic tiles which are usually ±3.5mm deep (but do check). MDF frames are only suitable for indoor use.